Cdc7 is a serine/threonine kinase that promotes DNA replication origin firing by phosphorylating one or more subunits of the MCM DNA helicase complex (Mcm2-7), thus leading to the unwinding of double-stranded DNA at origins of replication. Like cyclin dependent kinases, Cdc7 activity requires the binding of either one of two regulatory subunits, Dbf4 and Drf1/Dbf4B. Periodic accumulation of Cdc7, Dbf4 and Drf1/Dbf4B during S-phase is thought to be the major mechanism that regulates Cdc7 activity during the cell cycle.
Cdc7 depletion through RNA interference causes tumor cells to enter apoptosis in a p53 independent manner, while simply arresting cell cycle progression in normal cells, Furthermore, Cdc7 is a downstream target of the replication checkpoint proteins ATR and Chk2 and it is therefore not only an essential cell cycle regulator but also important for genome integrity in response to DNA damage. As a consequence, Cdc7 depletion in the presence of topoisomerase inhibitors or intercalating agents, increases cell death.
Altered expression of proteins involved in the initiation of DNA replication closely correlates with aggressive phenotypes and is a powerful marker of clinical outcome in a variety of malignancies. Cdc7 levels are increased in many cancer cell lines and primary tumors, such as breast, lung, ovary and melanoma cancers, compared to matched normal tissues and correlate with poor prognosis. Furthermore, somatic CDC7 mutations have been identified in colorectal and gastric carcinomas through comprehensive kinome screens of human tumors, DBF4 is also considered as a novel determinant in cutaneous melanoma development with prognostic relevance and it is also found to be amplified in some tumor cell lines and primary tumors, such as colon, lung and ovary.
These findings suggest that alterations in Cdc7/Dbf4 protein abundance or activity may occur during tumorigenesis and have important consequences for cell survival.
Drugs that target DNA replication elongation are widely used in chemotherapy, for example, gemcitabine, active metabolites of 5-fluorouracil and hydroxyurea, topoisomerase inhibitors, or DNA intercalating agents. A blockade of replication forks often results in breakage of the DNA molecules, and in the activation of an ATR/ATM dependent S-phase checkpoint pathway that senses the damage and mediates cellular responses to drug treatment. In contrast, Cdc7 inhibition prevents the activation of replication origins but does not trigger a sustained activation of the DNA damage response and a cell cycle block but rather induces apoptosis.
Some heteropentacycles have been demonstrated to be potent inhibitors of Cdc7 and are thus useful in the treatment of the proliferative disorders, especially cancer. One of these compounds is currently in development as an anti-cancer agent.
There is a continuous need of anticancer agents in order to optimise the therapeutic treatment. The present invention fulfils this need by providing new combinations of a Cdc7 inhibitor with known pharmaceutical agents that are particularly suitable for the treatment of proliferative disorders, especially cancer. More specifically, the combinations of the present invention are very useful in therapy as antitumor agents and lack, in terms of both toxicity and side effects, the drawbacks associated with currently available antitumor drugs.